I'm going to show you tips and tricks for doing math in your head. Doing division in your head can be difficult, but you always have to remember to simplify the problem. Let's take 2187/36 as our example. Now, we want to simplify this, so let's go ahead and look for what we can divide these numbers by. We know 36 is divisible by 3, but is 2187? There's an easy trick to find out. Write down all the numbers that make up 2187, 2, 1, 8, and 7, and add them together. 2+1+8+7 is an easy problem and it equals 18. Add those numbers together and you'll get 9. We simply check if this result is divisible by 3. 9 is divisible, so we know that 2187 is divisible by 3. Let's work that equation out. We'll come up with 729. When you divide the other side of the problem by 3, 36/12. Now, we have 729/12. A little bit easier, but still too much to work with. Let's try one more time. We know 12 is divisible by 3. What about 729? We use the same trick again. 7+2+9=18. 1+8-9. 9 is divisible, so 729 is divisible by 3. Go ahead and do that problem, and you'll come up with 243. Divide the other side of the equation by 3, and you'll come up with 4. Now, we're left with 243/4, a far simpler equation than what we started with. We can calculate this one out, and we'll come up with 60.75, which is our answer. Whenever you're doing math in your head, the most important part is being able to visualize the numbers. The more you practice, the better you're going to get.